The meaning would be different. If you say 'a clear blue sky', you are referring either to the sky at one particular moment, or - idiomatically - to an unexpected event.
There was clear blue sky throughout the journey.
There was a clear blue sky when we set out, but the clouds got thicker and thicker as we neared the camp-site, and we had to put the tent up in pouring rain.
We were getting on fine, when out of a clear blue sky she suddenly started crying.
- For Teachers